Added static value for the default API key

To make it less confusing for the users.
Added new action: search
Added argument for searching by type (post/thread)
Fixed search and added new type "thread_title"

Closes #28
Started playing with session/cookies

Working on implementing the features for issue #23.

It's now possible to start a XenForo session by using the "login" action
and then setting a cookie on the client's browser with the returned
cookie data, this will automaticlly log the user in when they visit the
XenForo forum.

I'll come up with an example later.
Added initial PHP wrapper for XenAPI

Still have to add all of the actions
Added message_html for getPost

This closes #25
Added HTML for getPosts and fixed bug

getPosts now returns HTML when grab_content is used.
getUser() returned FALSE instead of NULL, which caused some errors when
using the API key instead of an user.
Added new action: getUserUpgrades

Working on feature #26
getUserUpgrades returns a list off all available user upgrades.
Added new action: getUserUpgrade

Working on feature #26
getUserUpgrade(ID) where ID is the ID of the user upgrade, returns the
information about the specific user ugprade.
Added new user argument for getUserUpgrades

For feature #26
Using the user= parameter now returns the list of active user upgrades
for an user, user id/username and user email works for this parameter.
Unserialize extra user upgrade data

For feature #26
Added new action: upgradeUser

For feature #26
This action enables upgrading users by using the upgradeUser parameter.
The API will return the upgrade record and if the upgrade already exists
or if it's a new upgrade.
The upgrade will get extended by the specified default upgrade time if
the upgrade already exists for the user and end_time is not specified.
Added new action: downgradeUser

For feature #26
Allows downgrading a specific user upgrade for an user.
Added methods for checking if a model/addon exists
Added support for Custom Fields by Waindigo

This closes #11.

getThread, getThreads, getResource and getResources will now return a
new field called custom_fields, which is either null or contains an
array of custom fields.

Addition to that getUser now also return the user fields (aim, facebook,
etc).
Added new action: editThread

This closes #22 and continues the work on #7.

Editing the initial message of the thread does not work yet, it'll have
to be added later on. Editing the node, title, prefix, open/close state
and sticky-state of the thread now works.
Added new action: editPost

This continues work on #7, still need to add editing of all the other
actions (profile posts, etc).
Added new action: deleteUser

This continues work on feature #6.
Merge branch 'master' into dev-1.4

Conflicts:
	net/xenapi/XenAPI/api.php