Allow empty logs after a fresh wal.Open

See #33

Include AllowEmpty option and IsEmpty method

This commit includes the AllowEmpty option which allows for
deleting all entries from the log using the TruncateFront and
TruncateBack methods.
Using AllowEmpty changes the behavior of the log as it relates
to FirstIndex and LastIndex in the following ways:

- An empty log will now always have the FirstIndex be equal to
  LastIndex+1. Witout AllowEmpty, FirstIndex can never be
  greater than LastIndex.
- For a newly created log that has no entries, FirstIndex() and
  LastIndex() return 1 and 0, respectively. Without AllowEmpty,
  both return 0.

Also in this commit:

- IsEmpty method, which returns true if the log has no entries
- More unit tests
- Various code cleanup

See #31

Merge pull request #25 from vadiminshakov/master

fix truncate err handling

Merge pull request #10 from ctrix/master

More secure and user defined file permissions

Upgrade modules

Updated gjson dependency

Merge pull request #3 from gaoxinge/fix

Fix: catch os.Rename error

Updated packages

Merge pull request #2 from dim/fix/lock-contention

Avoid read-lock contention

Fix NoCopy