Add more referencing options

This adds a base function 'apply_reference' that can apply various
different referencing schemes. Two convenience functions are also
added that provide a convenient interface to the base function:

 - set_eeg_reference: apply a simple reference to all EEG channels
 - set_bipolar_reference: apply a bipolar reference (or more)

A helper function `_parse_channel_list` is also added to help functions
be more flexible in how they allow channels to be specified.