Remove dataframe identity test until I write better testdata generation stuff
Rewrite perfdata parsing to be less crap in various ways

For instance:

 - parse things that should be ints as ints instead of floats
 - don't throw away empty values
 - parse units of measurement instead of throwing them away
 - multiplying Nagios-defined units for time and data to a normalized
   form (microseconds and bytes, respectively)
Document some more things