Prettify inlinetasks in odt and (x)html backends

Prettify inlinetasks in odt and (x)html backends

* lisp/org-inlinetask.el (org-inlinetask-export-templates):
Fixed template for html so that the exported file is valid
xhtml. Added template for odt.
(org-inlinetask-export-handler): Fix typo in the regexp that
trims content. Make sure that the content is flanked by
paragraph boundaries on either side.

* lisp/org-html.el (org-export-html-style-default): Add style
for inlinetask.

* contrib/lisp/org-xhtml.el (org-export-xhtml-style-default):
Add style for inlinetask.

* contrib/odt/styles/OrgOdtStyles.xml: Added following custom
styles:  OrgInlineTaskHeading and OrgInlineTaskFrame.

* contrib/lisp/org-odt.el (org-odt-format-textbox)
(org-odt-format-inlinetask): Added

Refer following threads:
1. http://lists.gnu.org/archive/html/emacs-orgmode/2011-08/msg00238.html
2. http://lists.gnu.org/archive/html/emacs-orgmode/2011-08/msg00154.html

Thanks to Suvayu Ali for reporting this issue and suggesting
improvements.