* Enable notification for accounts change
* Fixed bugs in extHostAccountManagement.test.ts
* Fixed as commented:
1. Removed AccountWithProviderHandle
2. Use a private dictionary _accounts in ExtHostAccountManagement to cache all accounts and corresponding provider handles.
3. getSecurityToken gets provider handle from _accounts for specified account.
4. Added / changed unit tests for getAllAccounts & getSecurityToken
* 1.Added following functions to namespace accounts
function getAllAccounts(): Thenable<AccountWithProviderHandle[]>;
function getSecurityToken(account: AccountWithProviderHandle): Thenable<{}>;
2.Added class AccountWithProviderHandle as the wrapper for Account
3.Changed function openConnectionDialog of namespace connection to allow connection dialog initialized with specified parameters, i.e., server name, database name, etc.
function openConnectionDialog(provider?: string[], initialConnectionProfile?: IConnectionProfile): Thenable<connection.Connection>;
* Added unit tests for ExtHostAccountManagement.$getAllAccounts
Vincent Feng