Make c_format_unix_time and c_format_unix_time_gmt return the total number of characters copied into the buffer.
Factor out memory allocation in formatUnixTime and formatUnixTimeGMT into a helper function. Change the maximum number of characters to be copied into the buffer to 80 and use the total number of characters copied to resize the buffer before building a ByteString.
Make formatUnixTime return an IO ByteString because it is not referentially transparent. fixes #12
Merge pull request #13 from krzysztofwos/master

Minor refactoring and memory allocation fixes plus change of formatUnixTime's signature to address referential transparency issue.