Revamp API (output type and more)

*) introduce output type as a parametric argument
   to FastaReader
*) default to ASCIIString output type rather than
   Vector{Uint8}
*) new readentry(::FastaReader) function
*) overload Base.eof(::FastaReader)
*) new field FastaReader.num_parsed to get how many
   entries were parsed so far
*) new tests for the new functionality
*) new examples for the new functionality
Added rst docs