BUG: correct next_revision, previous_revision methods

`Revision.next_revision` and `Revision.previous_revision` does not work correctly.

The methods implementation is particularly reverted as in previous

Bug raised at: 35f3395c2eaf149e4d2392606a24ac237a753c26
Previous implementation can be found at: d9f6a859d726b164e584047c3436d89c2ae54c79
Merge pull request #161 from ksurya/fix-pag

BUG: correct next_revision, previous_revision methods
ENH: auto expand ace-editor height

1. As the code length increases, the height of editor expands automatically.
ENH: Ajax reST preview for new submissions

1. add `scipy_central.rest_comments` module to `INSTALLED_APPS`
2. rewrite `rest_comments.views.rest_to_html_ajax` view
3. `/static/rest_comments/rest-actions.js` a generic reST interface to convert content. `scipy_central.comments` module can be ported to it.
4. Ajax previewer for `description` field for new submissions
BUG: remove `item_highlighted_code` field in sample data

`item_highlighted_code` field no longer used in `submission.models.Revision`.
BUG: remove redundant classes from HTML

`spc-markup-help`, `spc-odd` and `spc-even` are no longer used
Merge pull request #171 from ksurya/ajax-preview

Ajax preview Form Submission
ENH: show upload-image form as text

1. instead of showing upload-image form as field with button,
show as a text attached to the bottom of description
Merge pull request #172 from ksurya/upload-img-btn

Change upload image button design
BUG: replace custom select-file button with default

custom select-file input button (tweaked) unable to show selected file name.
the default input button is restored.
Merge pull request #173 from ksurya/fix-select-file

Fix select file button
Merge pull request #174 from ksurya/sample-data-fix

BUG: remove `item_highlighted_code` field in sample data
Merge pull request #175 from ksurya/maint-code

BUG: remove redundant classes from HTML
ENH: rewrite core functionality of submission app

1. Remove `submission.models.ZipFile`
    ZIP files are no more staged
2. Rewrite `submission.forms.py` using model forms
    a. `submission.models` now contains help text, labels for form fields
    NOTE: Making keywords bold to show in HTML is now not done.
3. Rewrite views corresponding to create, edit submissions using class based views
    Removed functions, views from views.py:
        a. `get_form()`, `create_or_edit_submission_revision()`
           `get_license_text()`, `new_or_edit_submission()`,
           `edit_submission()`
    New view handlers:
        a. `BaseSubmission()`, `NewSubmission()`, `EditSubmission()`
        Some of the functions are still retained
        NOTE: New view handlers no longer provide previewing functionality from server. Rather it is now handled with JavaScript
4. Create new module `views` with `create.py`, `show.py`
    a. `create.py` contain views to create/edit submission
    b. `show.py` contain views to display
5. Create new module `submission.storage` to handle storing of submissions in filesystem.
6. Add new methods to `filestorage.dvcs_wrapper.py`
    a. `remove()`, `purge()`, `update()`
    NOTE: Needs to turn on `purge` extension in Mercurial (Hg) repository settings in `.hgrc` file on server

Minor Changes:

1. URL Conf for new submission is changed
    from: `r'^new/(?P<item_type>[a-zA-Z]+)/$`
    to  : `r'^(?P<item_type>[a-zA-Z]+)/new/$'`
2. submit button in `submission/new-item.html` is enhanced to disable on submitting the form to server using JavaScript
3. `person.views.profile_page_edit` view's `item` context is renamed to `form` to support default class based views context in `submission/new-item.html`
Merge pull request #168 from ksurya/new-sub

Improving submission module