[mcp-account-manager] Delay creating accounts until enabled for the first time

If a telepathy account is created immediately for a new account, the
telepathy property writes can race against other applications and result
in those properties being lost. Additionally, some properties of the
account may not be properly set at the instant when the account is
created. The enabling of the account is a good signal that it is fully
created and ready to use.
Merge pull request #12 from special/account-enabled

[mcp-account-manager] Delay creating accounts until enabled for the first time
Revert "[mcp-account-manager] Delay creating accounts until enabled for the first time"

This reverts commit 4739efef78b52d228bfa8450dbda9fde4a073db3. New
crashes are showing up in mission-control after this patch.
Revert "Revert "[mcp-account-manager] Delay creating accounts until enabled for the first time""

This reverts commit 0284b500bb5d64b889a4f5768bdd9d46c41735c8. The
crashes that led to this revert turned out to be coincidentally timed
and unrelated.