Tightened account handling; Front end uses PKs in DDLAccount box;

CHANGED: Trading.aspx
-- Removed hfAccounts field.

CHANGED: Trading.aspx.cs
-- Removed population of hfAccounts field.
-- Now populates DDLAccount with Account PK's as indices.

CHANGED: UserAccountManager.cs
-- Can now retrieve Account objects using GetPrivilegedAccounts()

CHANGED: AccountManager.cs
-- Added Exists() fn to check if the account exists.
-- Now will not create account with empty name.
-- Now returns the account whether valid or not.

CHANGED: AccountDAL.cs
-- Now warns if duplicate account names exist, and does not load them.

CHANGED: Account.cs
-- Comments