one potential fix for #73

**DO NOT MERGE!!**

I am exploring possibly options for solving this. 

Not sure i like this solution. It feels ugly, but will work as follows:

```
In [10]: reload(s)
Out[10]: <module 'bipy.core.sequence' from 'bipy/core/sequence.py'>

In [14]: s.RNA('ACU').rc()
Out[14]: <RNASequence: AGU (length: 3)>

In [15]: s.rna_rc('ACU')
Out[15]: 'AGU'

In [16]: s.rna_rc('ACT')

 ...snip...

BiologicalSequenceError: Don't know how to complement base T. Is it in
RNASequence.complement_map?

In [17]: s.dna_rc('ACT')
Out[17]: 'AGT'

In [18]: s.DNA('ACT').rc()
Out[18]: <DNASequence: AGT (length: 3)>
```