Merge pull request #2 from bennAH/master

added publish async
return errors if present on subscribe functions
fix type in error return
Rename channel to topic

This commit renames the misleading wording for channel to
topic, also a common denominator in this domain.
Use defer for unlock

This commit introduces the common idiom of using defer
for releasing locks. This will make the code slightly more
readable and less error prone.
Merge pull request #3 from bennAH/master

add document for func PublishAsync
Merge pull request #4 from briandowns/master

return errors if present on subscribe functions
add error return to unsubscribe
Merge pull request #7 from briandowns/master

add error return to unsubscribe
Merge with branch 'gittex-topic'
Merged #5
Merged #6
Merge branch 'gittex-defer' #6
Fixed missing return statement
Update README.md
Update README.md
Sample tests
Travis config