Use "equivalence" for all dictionary equality checks

This should fix a bug @mgarvert encountered with concatenating variables with
different array attributes.

In the process of fixing this issue, I encountered and fixed another bug with
utils.remove_incompatible_items.
Merge pull request #144 from shoyer/equivalent

Use "equivalence" for all dictionary equality checks
Remove install_requires if on readthedocs

This is the simplest solution I can think of for my documentation build woes.
More stupid fixes for readthedocs
Merge pull request #145 from xray/fix-rtfd

Fix doc builds on ReadTheDocs
Support "None" as a variable name and use it as a default

This makes the xray API a little more similar to pandas, which
makes heavy use of `name = None` for objects that can but don't always
have names like Series and Index.

It will be a particular useful option to have around when we add
a direct constructor for DataArray objects (#115). For now, arrays will
probably only end up being named `None` if they are the result of some
mathematical operation where the name could be ambiguous.
Fix Python 3 failures comparing None to strings
Merge pull request #147 from shoyer/variable-None

Support "None" as a variable name and use it as a default
Constructor to create DataArrays from scratch
Switch DataArray to use __init__ instead of __new__

__new__ is no longer necessary because we can consolidate all the initailization
in a single method, and no longer need to warn about direct usage.
Copy dataset when creating a DataArray?

This would make it safe to modify a DataArray in-place.
Faster Dataset.copy() by avoiding validation
New data_array.Indexes object allows for name and int based indexing
DataArray constructor now infers arguments if data is a self-described array
Coordinate.as_index now includes the name
Infer indexes from pandas objects when constructing a DataArray
Fix DecodedCFDatetimeArray was being incorrectly indexed.
Fix bug in test_conventions
Merge pull request #150 from akleeman/decode-cf-slicing-bug

Fix DecodedCFDatetimeArray was being incorrectly indexed.
load_data() method for trigger loading of data into memory

Fixes #146