Updated instruction mnemonics

These instruction mnemonics have been updated to conform to IEC 61131-3 and PLCopen standards.

Instruction	Mnemonic in versions 1.05 and earlier	Mnemonic in versions 1.06 and later
Arc Cosine	ACS	ACOS
Arc Sine	ASN	ASIN
Arc Tangent	ATN	ATAN
Convert to BCD	TOD	TO_BCD
Convert to Integer	FRD	BCD_TO
Equal To	EQU	EQ
Greater Than	GRT	GT
Greater Than or Equal To	GEQ	GE
Less Than or Equal To	LEQ	LE
Less Than	LES	LT
Limit	LIM	LIMIT
Move	MOV	MOVE
Not Equal To	NEQ	NE
Square Root	SQR	SQRT
Truncate	TRN	TRUNC
X to the Power of Y	XPY	EXPT