alternative strategy to fix #73

**DO NOT MERGE**

This is an alternative to #80 to fix #73. Will work as follows:

```
In [44]: reload(s)
Out[44]: <module 'bipy.core.sequence' from 'bipy/core/sequence.pyc'>

In [45]: s.rna_complement('ACU')
Out[45]: 'UGA'

In [46]: s.rna_rc('ACU')
Out[46]: 'AGU'

In [47]: s.RNA('ACU').complement()
Out[47]: <RNASequence: UGA (length: 3)>

In [48]: s.RNA('ACU').rc()
Out[48]: <RNASequence: AGU (length: 3)>

In [49]: s.RNA.comp('ACU')
Out[49]: 'UGA'

In [51]: s.RNA.rev_comp('ACU')
Out[51]: 'AGU'
```