Implement the Erbs model (#166)

* add erbs

* tweak doc string

* tweak doc string

* add scalar input option

* doc fix

* make math eqns a little nicer

* add 0.3.3 whatsnew

* add appveyor to bug fixes

Author: wholmgren

docs/sphinx/source/whatsnew.rst

@@ -6,6 +6,7 @@ What's New
 
 These are new features and improvements of note in each release.
 
+.. include:: whatsnew/v0.3.3.txt
 .. include:: whatsnew/v0.3.2.txt
 .. include:: whatsnew/v0.3.1.txt
 .. include:: whatsnew/v0.3.0.txt

docs/sphinx/source/whatsnew/v0.3.3.txt

@@ -0,0 +1,25 @@
+.. _whatsnew_0330:
+
+v0.3.3 (May xx, 2016)
+-----------------------
+
+This is a minor release from 0.3.2.
+We recommend that all users upgrade to this version.
+
+Enhancements
+~~~~~~~~~~~~
+
+* Adds the Erbs model. (:issue:`2`)
+
+
+Bug fixes
+~~~~~~~~~
+
+* Fix another bug with the Appveyor continuous integration builds.
+  (:issue:`170`)
+
+
+Contributors
+~~~~~~~~~~~~
+
+* Will Holmgren