Fix failure when creating votables

The following code
```
from astropy.io.votable.tree import VOTableFile, Resource, Table, Field
votable = VOTableFile()

resource = Resource()
votable.resources.append(resource)

table = Table(votable)
resource.tables.append(table)
table.fields.extend([Field(votable, name='XXX', datatype="float", unit = "mag")])
```

fails with

```
ERROR: KeyError: 'unit_format' [astropy.io.votable.tree]
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-28-1bdee196c715> in <module>()
----> 1 table.fields.extend([Field(votable, name='XXX', datatype="float", unit = "mag")])

/data/guenther/anaconda/lib/python2.7/site-packages/astropy-0.3.dev4321-py2.7-linux-x86_64.egg/astropy/io/votable/tree.pyc in __init__(self, votable, ID, name, datatype, arraysize, ucd, unit, width, precision, utype, ref, type, id, xtype, config, pos, **extra)
   1152         self.arraysize  = arraysize
   1153         self.ucd        = ucd
-> 1154         self.unit       = unit
   1155         self.width      = width
   1156         self.precision  = precision

/data/guenther/anaconda/lib/python2.7/site-packages/astropy-0.3.dev4321-py2.7-linux-x86_64.egg/astropy/io/votable/tree.pyc in unit(self, unit)
   1323                           self._config, self._pos)
   1324
-> 1325         format = _get_unit_format(self._config)
   1326         if format != default_format:
   1327             unit_obj = u.Unit(

/data/guenther/anaconda/lib/python2.7/site-packages/astropy-0.3.dev4321-py2.7-linux-x86_64.egg/astropy/io/votable/tree.pyc in _get_unit_format(config)
    180     Get the unit format based on the configuration.
    181     """
--> 182     if config['unit_format'] is None:
    183         format = _get_default_unit_format(config)
    184     else:

KeyError: 'unit_format'
```

There is any easy fix for this (and I have implemented it in this pull request); I have also written a unit test.
However, this implementation breaks a different test when reading unrecognized units and I have been unable to figure out why that is.
modified as per suggestion in github discussion
Put astropy.io.votable.writeto in the top-level namespace
Don't use filecmp to compare these files--it gives different results on Windows due to default newlines being different.  Instead just compare the content.
Merge pull request #1102 from hamogu/votabmakeunits

Fix failure when creating votables
Merge pull request #1101 from mdboom/vo/writeto

astropy.io.votable: no write_to method
Started documenting some details on how I maintain the bug fix branch(es).  Also added some details on how cherry-picking merge commits works, adapted from an e-mail I wrote some months ago.  Still need to describe the steps for preparing a bug fix branch for a release.
Merge pull request #1103 from iguananaut/validator-issues

Fix vo.validator tests on Windows
Mostly finished the dicussion of maintaining bug fix releases, and modified the release procedure to account for them.  There is a mention of 'updating the changelog' that is left hanging, but I will try to address that in #695.
Use repr() to format strings instead of putting literal quotes in the format string.
Adds directions for updating the changelog (and some justification for why it exists and why you should add to it)
Merge pull request #1104 from iguananaut/sphinx-windows-issues

Fix minor issue with build_sphinx on Windows
Fixed bug in WCS error message
Merge pull request #1108 from astrofrog/wcs/fix-error-message

Fixed bug in WCS error message
Merge pull request #940 from iguananaut/issue-940

Document releasing bug fix releases
Merge pull request #695 from iguananaut/issue-695

Write/improve guidelines for contributing the the CHANGES.rst changelog