Fix golangci#122: don't crash govet on tests-only packages

Fix golangci#94: load object files for govet for old go

Do it in compatible with old go versions object files way:
use golang.org/x/tools/go/gcexportdata instead of importer.Default

Fix golangci#17, golangci#87: govet becomes SLOW linter by default

1. Allow govet to work in 2 modes: fast and slow. Default is slow.
In fast mode golangci-lint runs `go install -i` and `go test -i`
for analyzed packages. But it's fast only when:
  - go >= 1.10
  - it's repeated run or $GOPATH/pkg or `go env GOCACHE` is cached
  between CI builds
In slow mode we load program from source code like for another linters
and do it only once for all linters.

3. Patch govet code to warn about any troubles with the type
information. Default behaviour of govet was to hide such warnings.
Fail analysis if there are any troubles with type loading: it will
prevent false-positives and false-negatives from govet.

4. Describe almost all options in .golangci.example.yml and
include it into README. Describe when to use slow or fast mode of govet.

5. Speed up govet: reuse AST parsing: it's already parsed once by
golangci-lint.
For "slow" runs (when we run at least one slow linter) speedup by
not loading type information second time.

6. Improve logging, debug logging

7. Fix crash in logging of AST cache warnings (golangci#118)

Merge pull request golangci#84 from golangci/support/detect-mockgen-p…

…roperly

Properly detect generated files: fix detection when

Merge pull request golangci#82 from golangci/feature/match-more-autog…

…enerated-files

Fix golangci#72: match more autogenerated files patterns.

Merge pull request golangci#71 from golangci/feature/full-support-of-…

…nolint-directives

golangci#65, golangci#68: make //nolint processing like in gometalinter

fix --version option and improve installation section of docs

Merge pull request golangci#62 from golangci/feature/tab-output-format

golangci#37: add tab output format: --out-format=tab

Merge pull request golangci#46 from golangci/support/fix-no-results-f…

…or-gocyclo

golangci#45: fix no results for gocyclo

Merge pull request golangci#44 from golangci/support/exit-code-4-if-d…

…eadline

golangci#41: exit with code 4 if timeouted