Replace Text backend with CSV

The main reason to keep the Text backend around was that pandas was an
optional dependency.  Now that this is no longer the case, the
original Text backend doesn't offer any advantages over the CSV
backend.  (Even if someone prefers not to write to files while
sampling, they can sample with the NDArray backend and then use the
CSV dump function.)

Rename CSV to Text.  This name is more appropriate because the values
being stored as plain text is the important feature, not which
delimiter is used.

Author: kyleam

pymc3/backends/__init__.py

@@ -4,11 +4,11 @@
 ------------------
 
 1. NumPy array (pymc3.backends.NDArray)
-2. Plain text formats (pymc3.backends.Text and pymc3.backends.CSV)
+2. Text files (pymc3.backends.Text)
 3. SQLite (pymc3.backends.SQLite)
 
-The NDArray and Text backend both hold the entire trace in memory,
-whereas CSV and SQLite backends store the values while sampling.
+The NDArray backend holds the entire trace in memory, whereas the Text
+and SQLite backends store the values while sampling.
 
 Selecting a backend
 -------------------
@@ -107,16 +107,13 @@
 If the traces are stored on disk, then a `load` function should also be
 defined that returns a MultiTrace object.
 
-For specific examples, see pymc3.backends.{ndarray,text,csv,sqlite}.py.
+For specific examples, see pymc3.backends.{ndarray,text,sqlite}.py.
 """
 from ..backends.ndarray import NDArray
 from ..backends.text import Text
-from ..backends.csv import CSV
 from ..backends.sqlite import SQLite
 
 _shortcuts = {'text': {'backend': Text,
                        'name': 'mcmc'},
-              'csv': {'backend': CSV,
-                      'name': 'mcmc'},
               'sqlite': {'backend': SQLite,
                          'name': 'mcmc.sqlite'}}